Those features (HR monitor, GPS, swimming metrics, etc) found on true fitness watches are expensive and bulky (most of them anyway). You can tell they'll get there eventually, but as much as I'd love to have GPS in the Apple Watch it's really not feasible for that design yet.
I've used (and continue to use) a few different watches for swimming, running, triathlons, etc. Didn't expect an Apple Watch to be able to replace those just yet.